WikiProject SoCal MTB
This is a project to map mountain bike trails in the Southern California area.
Coverage of fire roads in the area already quite good from The TIGER data. Many fire roads are unmaintained and are now singletrack. Re-tagging these paths should be a good start.
GPS units have gained enormous popularity among trail riders. Soliciting input from the community, it should be possible to get good coverage of all the popular trails.
An eventual goal would be to improve rendering of maps for mountain bike use.